The present invention relates in general to sewing machines and, in particular to a new and useful method and device for forming overcast seams using a standard zig-zag sewing machine.
From German OS No. 28 11 455 is known a method for the formation of a seam that can be used as an edge finishing seam on a zig-zag sewing machine, according to which the third of three forward stitches is made by changing the lateral stitching position. The first of the four forward stitches is made by changing the stitch position, and the following two are made in this stitch position. The fourth is made again by changing the stitch positon, and so on whereby the stitches spaced from the cloth edge are sewn by reversing the direction of feed. In this way needle and bobbin thread are formed concatenated at each penetration of the needle, both on the outer edge of the cloth and at a distance thereof, hence in both overstitch positions of the needle. All overstitches are made in the direction of feed.
Since this seam is only formed by controlling the overstitch movements of the needle and the feeding movements of the cloth feed, so that it is a pure zig-zag seam, it has the inconvenience inherent in all known zig-zag seams namely, that it must be sewn with a much lower tension of needle and bobbin thread than a straight seam, in order to keep the unavoidable unsightly gathering of the seam, and thus of the sewing material at a minimum, which is particularly conspicuous in the production of an edge finishing seam. But this cannot be completely avoided in the production of a zig-zag seam if an orderly and still sufficiently firm concatenation of needle-and bobbin threads is to be achieved by tying the stitch.
When changing from sewing a zig-zag seam to sewing a straight seam and vice versa, a readjustment of the tension of needle and bobbin thread cannot be avoided. Readjustment of the tension is mostly not possible the first time, but only after a few tension changes and subsequent sewing tests. This means that considerable time is lost.
In the production of festoon and fancy seams, it is known to use a twin needle sewing machine (German Pat. No. 285,182), which is equipped with a thread puller by which the thread of one needle is pulled out and offered to the other needle for penetration in order to avoid bending stresses of the needle. The control of the thread puller requires a relatively complicated fixer gear which is installed in the machine. The machine becomes thus a special machine for the production of two parallel seams where the thread of one needle is merely looped with that of the other needle in the manner of a festoon. As a single purpose machine, it is unsuitable for use in private households.